Measurement-While-Drilling (MWD) or Logging-While-Drilling (LWD) applications use a mud-pulse system of telemetry to create acoustic signals in the drilling fluid that is circulated under pressure through the drillstring during drilling operations. Information acquired by downhole sensors is transmitted by suitably timing the formation of pressure pulses in the mud stream. This information is received and decoded by a pressure transducer and computer at the surface. Typically, these systems have involved a valve and a control mechanism known as a pulser or a mud pulser. Operation of the valve sends a pulse up the drillstring at the velocity of sound in the drilling mud. The rate of transmission of data is relatively slow due to pulse spreading, distortion, attenuation, modulation rate limitations, and other destructive forces such as ambient noise in the drillstring. The mud pulser generates digital 1's and 0's, depending on whether it is open or closed. One prior attempt to increase the data rate is to increase the frequency of the pulses. However, increasing the pulse frequency makes it more difficult to distinguish between adjacent pulses because of short resolution periods.
Negative pulsing systems employ a valve which temporarily allows flow from the drill collar into the annulus, thus bypassing the bit. These systems have a disadvantage of taking flow away from the bit. Positive pressure pulse systems have been- created by temporarily restricting the downward flow of drilling fluid by partial blocking of the fluid path in the drillstring. Pulse detection at the surface can sometimes become difficult due to attenuation and distortion of the signal and the presence of noise generated by the mud pumps, the downhole mud motor, and elsewhere in the drilling system. The presence of grit and other particles in the mud also creates certain operational problems for transducers in the drillstring. Both the positive and negative mud pulse systems generate base band signals. A desirable objective to increase the transmission rate of data is to provide an increased band width signal in the form that provides easy delineation at the surface of the well.
In the past, mud pulse systems that transmit mud pulse signals of differing amplitudes have been developed. In one design, a poppet and orifice structure uses a configuration which provides a tendency for the poppet to remain in the closed position. A bypass line is provided around the poppet and orifice and to a driving piston on the poppet. The poppet valve opens by a pilot valve connected on the bypass conduit of the piston assembly. When the pilot valve turns off, mud flow is blocked through the piston assembly. Relief valves are provided in the bypass conduit downstream of the piston.
These relief valves are pre-calibrated to a particular pressure level which causes each valve to leak mud to prevent the predetermined pressure level from being exceeded. Thus, use of a variety of relief valves allows for the creation of a pressure pulse with an independent amplitude. This system and variations thereof are described in detail in U.S. Pat. No. 5,586,084.
However, this system suffers from various disadvantages. The control that it provides over the movements of the poppet are, at best, indirect. Through the use of the bypass line, the movements of the poppet are controlled by an applied hydraulic pressure acting in conjunction with a spring force. The physical movements of the poppet are not measured; thus, when the relief valve or valves selected reach their predetermined release pressure, the specific amplitude of the pulse generated is uncertain. This is also because erosion on the orifice or poppet affects the amplitude of the pulse generated and the control system described in U.S. Pat. No. 5,586,084 has no provisions for compensation for such erosion effects. Additionally, the use of bypass passages in drilling mud service also creates potential plugging problems in the small components, which would undermine the effectiveness of that system. The system of the prior art thus requires the use of many relief valves or a motor-driven variable restrictor which further presents operational difficulties in mud service. These components must be calibrated for the poppet and orifice combination in its new condition and cannot respond effectively to effects of erosion or dramatic differences in mud flow rates and operating pressures.
It is an object of the present invention to provide a mud pulser whose position is, directly set in response to measured pressure uphole in the drill-string. Another objective of the present invention is to be able to obtain greater precision in the amplitude of the pulses generated by sensing not only the measured pressure, but also its rates of increase. Another objective is to use the measured pressure from the pulses generated to translate directly to physical movement of the mud pulser to obtain greater control of the pulse amplitudes generated. Another objective is to be able to create baseline amplitudes and to maintain such amplitudes despite changing physical conditions of the mud pulser or in pressure and flowrates of the mud circulating through the drillstring. These and other advantages of the present invention will become more apparent to those skilled in the art from a review of the preferred embodiment described below.